Not seeing any other benchmarks, I decided to test this.
Fairly standard box - P4, ata_piix, 1G memory. Stock desktop install, fully
up to date with updates and updates-testing as of this afternoon.
A 'normal' boot to gdm is about 56.9 seconds. I installed
prcsys, and edited the startup scripts to add LSB dependencies
as attached.
I then booted with prcsys and parallel init. The new boot time was...

- how would this be useful for the case where facilities that are provided
are determined at runtime (say, NetworkManager providing $network instead
of /etc/init.d/network, or $remote_fs being provided by either rc.sysinit
or /etc/init.d/netfs, depending on configuration).

yep, tbd
>> Similarly, you may

want a meta-dependency for 'authorization available', which would be
at different times depending on whether or not you're using local
passwords, KRB5, etc.